1. Filed of the Invention
The present invention relates to a color adjustment method for color-adjusting a color image for a color printer, color copying machine or the like.
2. Description of the Related Art
As a method for color adjustment of a color image have hitherto been proposed the two following systems:
(1) a system in which the color of the whole of an image to be adjusted is adjusted in reference to a specified color in the image to be adjusted; and PA1 (2) a system in which the color of a specified area (or display space area or color space area) in an image to be adjusted is adjusted.
However, the system (1) has the problem of influence on another color in the image and the system (2) has problems including the complexity of designation of the specified area, the generation of pseudo outlines and so forth.
An improvement on the systems (1) and (2) has been proposed by "A method of color correction by corresponding to plural desiable colors", Proceedings of COLOR FORUM JAPAN '94, pp. 19-22. This improved system is a color adjustment system in which a processing for the whole of an image to be adjusted is performed in such a manner that a plurality of colors in the image to be adjusted (designated colors) and colors desired to obtain after adjustment corresponding to the designated colors (adjustment colors) are given and each correspondence is extrapolated smoothly over the whole of a coordinate system in which a color adjustment processing is performed.
A procedure for color adjustment processing by the improved system includes the three following steps.
Namely, a portion having colors to be subjected to color adjustment (designated colors) is selected from an inputted image to be adjusted so that a color desired to obtain after adjustment (adjustment color) is caused to correspond to each designated color (step 1).
An extrapolation calculation on condition of the correspondence of the designated color to the adjustment color is made to define a function having output values for all input values in a coordinate system in which a color adjustment processing is performed (step 2).
The function defined by the extrapolation calculation is used to perform the color adjustment processing for all pixel data of the image to be adjusted (step 3). In actuality, inputted image data is converted to the value of the coordinate system beforehand and the adjustment processing is performed by use of a processing function. Thereafter, the data is converted again into data of a coordinate system at the time of input and the converted data is outputted.
This improved system uses, as the coordinate system for color adjustment, a L* Cuv* Huv.degree. (hereinafter abbreviated as LHC) coordinate system defined in a CIE-L* u* v* space which is one of uniform perceptual color spaces. The color adjustment of the whole of the image is realized by performing interpolation processings with an extrapolation function of luminosity L having a preference for a chroma C direction, with an extrapolation function of chroma C having a preference for a hue H direction and with an extrapolation function of hue H having a preference for a luminosity L direction.
However, the conventional color adjustment method has the following problem. Namely, in the case where a pixel having a color near to a first designated color on a color space is to be adjusted, the color image may be color-adjusted in an undesirable direction since notwithstanding that the color of that pixel is near to the first designated color, it is affected by color adjustment parameters for a second designated color a specified axis (for example, luminosity L) of which is near to that of the first designated color.